1. Field of the Invention
The invention relates to a liquid crystal display (LCD), and more particularly to an in-plane switching (IPS) type active matrix liquid crystal display.
2. Description of the Related Art
In general, a liquid crystal display is characterized by a thinner body, a lighter weight, and smaller powers consumption. In particular, an active matrix liquid crystal display (AM-LCD) where pixels arranged in a matrix are actuated by an active device is now expected as a flat panel display providing qualified images. Among active matrix liquid crystal displays, a thin film transistor type liquid crystal display (TFT-LCD) draws attention which employs a thin film transistor (TFT) as an active device for switching individual pixels.
A conventional active matrix liquid crystal display makes use of twisted nematic (TN) type electro-optical effect. That is, a conventional active matrix liquid crystal display applies an electric field almost perpendicular to substrates to liquid crystal sandwiched between the substrates, to thereby actuate liquid crystal.
An in-plane switching type liquid crystal display where liquid crystal is actuated by an electric field almost parallel to substrates has been suggested in U.S. Pat. No. 3,807,831. The suggested liquid crystal display employs an electrode including a plurality of sinking combs which are in mesh each other.
Japanese Patent Publication No. 63-21907 has suggested an active matrix liquid crystal display making use of twisted nematic electro-optical effect, which includes an electrode including a plurality of sinking combs in mesh each other for lowering a parasitic capacitor formed between a common electrode and a drain bus line, or a parasitic capacitor formed between a common electrode and a gate bus line.
FIG. 1 illustrates an above-mentioned conventional in-plane switching type liquid crystal display, which is comprised of a pair of glass substrates 11 and 12, a liquid crystal layer 20 sandwiched between the glass substrates 11 and 12, comb electrodes 70 formed on the glass substrate 11 and having sinking combs in mesh with each other, and a pair of polarizing plates (not illustrated) each positioned outside the glass substrate 11 or 12.
By applying a voltage across the comb electrodes 70, there is generated a liquid crystal driving electric field E1 parallel to surfaces of the glass substrates 11 and 12, and perpendicular to a direction in which sinking combs of the electrodes 70 extend. The thus generated electric field E1 changes orientation azimuth of liquid crystal molecules 21. Hence, it is possible to control transmissivity of optical beams by varying a voltage to be applied across the comb electrodes 70.
In such an in-plane switching type liquid crystal display as illustrated in FIG. 1, it is necessary to cause the liquid crystal molecules 21 to rotate only in a direction, when a voltage is applied thereto, in order to enable stable and uniform display. To this end, the liquid crystal molecules 21 are arranged to have initial orientation azimuth slightly deviating from a direction perpendicular to a direction of the liquid crystal driving electric field E1. In other words, now suppose that liquid crystal has initial orientation azimuth .phi.LC0 to be measured on the basis of a direction perpendicular to a direction in which parallel electrode pairs comprised of sinking combs of the electrodes 70 extend, the orientation of the liquid crystal molecules 21 is arranged so that the initial orientation azimuth .phi.LC0 is smaller than 90 degrees (.phi.LC0&lt;90.degree. ). Hereinbelow, in this specification, a direction of an electric field and orientation azimuth of liquid crystal are measured on the basis of a direction perpendicular to a direction in which parallel electrode pairs comprised of sinking combs of the electrodes 70 extend (.phi.=0). A counterclockwise direction is defined as positive.
As mentioned later, it is necessary in the in-plane switching type liquid crystal display illustrated in FIG. 1 to rotate the liquid crystal molecules 21 by 45 degrees relative to initial orientation azimuth thereof in order to accomplish adequate display contrast. To this end, it is preferable to arrange the liquid crystal molecules 21 to have orientation so that the initial orientation azimuth .phi. LC0 is equal to or greater than 45 degrees, but smaller than 90 degrees (45.degree..ltoreq..phi. LC0&lt;90.degree.).
In the liquid crystal display illustrated in FIG. 1, since the initial orientation azimuth of liquid crystal slightly deviates in a clockwise direction from a direction in which the parallel electrode pairs extend, when viewed from above the glass substrate 12, the liquid crystal molecules rotate in a direction shown with an arrow A, when a voltage is applied across the electrodes 70.
In the liquid crystal display illustrated in FIG. 1, optical transmissivity T is expressed with the following equation (1), if polarizing plates (not illustrated) are designed to have polarizing transmissive axes (namely, polarizing directions) perpendicular with each other. EQU T=(sin 2A.times.sin 2B)/2 EQU A=2(.phi.P-.phi.LC), B=.pi..DELTA.n d/.lambda. (1)
In the equation (1), .phi.LC indicates orientation azimuth of the liquid crystal molecules 21 when a voltage is applied across the electrodes 70, .phi.P indicates azimuth of a transmissive axis of a polarizing plate through which optical beams are introduced, .DELTA.n indicates index anisotropy of liquid crystal, "d" is a thickness of a cell, namely, a thickness of the liquid crystal layer 20, and .lambda. indicates a wavelength of optical beams.
Azimuth .phi.A of a transmissive axis of a polarizing plate through which optical beams leave is expressed as .phi.A=.phi.P+90.degree., or .phi.A=.phi.P-90.degree.. In accordance with the above-mentioned equation (1), the orientation azimuth (.phi. LC) of liquid crystal is varied by the liquid crystal driving electric field E1 parallel to the substrates 11 and 12, to thereby control the transmissivity of optical beams.
If one of the polarizing plates is designed to have a transmissive axis having a direction coincident with initial orientation azimuth of liquid crystal (.phi. LC0=.phi.P or .phi.LC0=.phi.A), dark state is generated when no voltage is applied across the comb electrodes 70, whereas the transmissivity is maximized when the orientation azimuth of liquid crystal is varied by 45 degrees by the liquid crystal driving electric field E1, and resultingly, bright state is generated. As an alternative, dark state can be generated when a voltage is applied across the comb electrodes 70, by differently arranging the polarizing plates.
In the explanation made so far, it was supposed for the purpose of simplifying the explanation that the liquid crystal molecules 21 in the liquid crystal layer 20 sandwiched between the glass substrates 11 and 12 rotate in a uniform fashion. However, liquid crystal molecules existing on interfacial surfaces of the glass substrates 11 and 12 are relatively strongly bonded to the substrates 11 and 12, and as a result, azimuth of such liquid crystal molecules remains almost unchanged.
In a display mode in which the above-mentioned birefringence effect is utilized, it is understood in view of the above-mentioned equation (1) that a light having a wavelength which meets the equation, .DELTA.n d=.lambda./2, can pass through the polarizing plate with highest efficiency. Hence, in order to accomplish multicolor display by using white-color display or a color filter, it would be necessary to arrange index anisotropy and a thickness of a liquid crystal layer so that transmissive spectrum has a principal wavelength of about 550 nm, that is, .DELTA.n d is equal to 275 nm (.DELTA.n d=550/2=275 nm). However, for such reasons as mentioned above, it is actually preferable to arrange .DELTA.n d to have a slightly greater value than the above-mentioned one, namely, arrange .DELTA.n d to have a value in the range of about 280 nm to about 330 nm.
Japanese Unexamined Patent Publication No. 5-505247 (International Publication No. WO91/10936) has suggested an improvement of viewing angle characteristic which is one of shortcomings of the above-mentioned in-plane switching type TN liquid crystal display. In these days, this superior viewing angle characteristic draws attention, and hence, an in-plane switching type active matrix liquid crystal display is applied to a large-sized monitor screen.
FIG. 2 illustrates a curve showing voltage-transmissivity characteristic in an in-plane switching type liquid crystal display. Herein, the voltage-transmissivity characteristic means how a relation between a voltage and transmissivity varies in dependence on an angle with which the liquid crystal display is viewed. An angle with which the liquid crystal display is viewed is defined with .phi.obs and .theta.obs where .phi.obs indicates an azimuth angle measured from a direction perpendicular to a direction in which an electrode extends, and .theta.obs is an inclination angle measured from a direction perpendicular to a substrate.
The liquid crystal cell used for the measurement was designed to have .phi.LC0 of 85 degrees, .phi.P of 85 degrees, and .phi.A of -5 degrees. The electrode had combs in mesh with each other, where each of the combs had a width of 5 .mu.m, and the combs were spaced away from each other by 15 .mu.m. The used liquid crystal material has index anisotropy of 0.067, and the cell had a thickness of 4.9 .mu.m.
As is understood in view of FIG. 2, fluctuation in a voltage-transmissivity characteristic caused by variation in a viewing angle is small in an in-plane switching type liquid crystal display, which means that an in-plane switching type liquid crystal display has superior viewing angle characteristic.
However, the above-mentioned in-plane switching type liquid crystal display has a problem that displayed images are unpreferably tinged with blue or red in certain viewing angles.
FIG. 3 illustrates how transmissive spectrum varies in bright state in dependence on viewing angles. A sample of the liquid crystal cell used for the measurement is the same as the liquid crystal cell used for the measurement of FIG. 2. In this liquid crystal cell, orientation azimuth of liquid crystal molecules varies by about 45 degrees, specifically, from initial orientation azimuth .phi.LC0=85 degrees where a voltage is not applied, to orientation azimuth .phi.LC in bright state where a voltage is applied. Accordingly, the orientation azimuth .phi.LC is equal to 40 degrees (85-45=40). As illustrated in FIG. 3, the liquid crystal cell in bright state has a tendency that a peak of transmissive spectrum deviates towards a shorter wavelength at .phi.obs=40 degrees, and resultingly, displayed images are tinged with blue.
On the other hand, a peak of transmissive spectrum deviates towards a longer wavelength at .phi.obs=-50 degrees, and resultingly, displayed images are tinged with red. The same tendency was observed at azimuth different by 180 degrees in a clockwise or counterclockwise direction.
FIGS. 4A and 4B illustrate loci of varying chromaticity, measured from transmissive spectrum obtained when a polar angle of an observation direction is fixed at 50 degrees, and an azimuth angle of an observation direction is varied from 0 to 360 degrees. FIG. 4A illustrates varied chromaticity in a state intermediate between dark and bright states, and FIG. 4B illustrates varied chromaticity in bright state.
As mentioned so far, an in-plane switching type liquid crystal display is superior to a conventional vertical field type or twisted nematic type liquid crystal display with respect to display contrast, gradation inversion, and so on. However, the above-mentioned problem that displayed images are tinged with blue or red in certain viewing angles remains unsolved.
In the above-mentioned liquid crystal cell, liquid crystal molecules are oriented at initial orientation azimuth .phi.LC0=85 degrees when a voltage is not applied across electrodes. When a voltage is applied across the electrodes to thereby generate bright state, the orientation azimuth .phi.LC of liquid crystal molecules varies by about 45 degrees from the initial orientation azimuth .phi.LC0. Hence, the orientation azimuth .phi.LC becomes equal to 40 degrees (85-45=40).
In FIG. 2, azimuth at which displayed images are tinged with blue corresponds to the orientation azimuth .phi.LC of 40 degrees, and azimuth at which displayed images are tinged with red is perpendicular to the orientation azimuth .phi.LC of 40 degrees. As mentioned earlier, the transmissive spectrum of the abovementioned liquid crystal cell is dependent on birefringence (.DELTA.n d) of a liquid crystal layer. The fact that displayed images are tinged with blue or red in certain viewing angles is based on dependency of apparent birefringence of a liquid crystal layer on a viewing angle. This is explained in detail hereinbelow.
Effective index anisotropy .DELTA.N obtained when a light obliquely enters the above-mentioned cell is defined with the following equation (2). EQU .DELTA.N=ne no/C.sup.1/2 -no EQU C=ne.sup.2 cos.sup.2 .theta..sub.2 +no.sup.2 sin.sup.2 .theta..sub.2(2)
In the equation (2), .theta..sub.2 indicates an angle formed between a direction in which a light is transmitted and a major axis of liquid crystal molecules, no indicates an index of refraction with respect to ordinary ray, which is a ray oscillating or polarizing in a direction perpendicular to an optical axis of crystal, namely, a direction perpendicular to a direction of a major axis of liquid crystal molecules, and ne indicates an index of refraction with respect to abnormal ray which is a ray oscillating or polarizing in parallel with the above-mentioned optical axis.
Since the angle .theta..sub.2 is equal to 90 degrees in perpendicular incidence, the index anisotropy .DELTA.N is given as .DELTA.N=.DELTA.n=ne-no. On the other hand, in a direction where displayed images are tinged with blue, since a viewing angle is inclined towards a major axis of liquid crystal molecules, the angle .theta..sub.2 is smaller than 90 degrees (.theta..sub.2 &lt;90.degree.), and hence, the index anisotropy .DELTA.N becomes smaller. In a direction where displayed images are tinged with red, since a viewing angle is inclined towards a minor axis of liquid crystal molecules, the angle .theta..sub.2 remains 90 degrees (.theta..sub.2 =90.degree.), and hence, .DELTA.N remains equal to .DELTA.n (.DELTA.N=.DELTA.n). FIGS. 5A and 5B illustrate how index anisotropy varies in dependence on a viewing angle.
Since a substantial thickness D of a liquid crystal layer is defined as D=d/cos(.theta.obs) in oblique incidence, the substantial thickness D of a liquid crystal layer becomes greater regardless of a direction to which a viewing angle is inclined.
As both the index anisotropy An and the thickness "d" of a liquid crystal layer vary, a birefringence (.DELTA.n d) varies, which in turn varies color with which displayed images are tinged in dependence on viewing angles.
The explanation made so far can be summarized as follows.
A direction where displayed images are tinged with blue is identical with a direction of a major axis of liquid crystal molecules in bright state, in which case, the index anisotropy .DELTA.n is reduced, and the thickness "d" of a liquid crystal layer is increased. However, since the degree of reduction in the index anisotropy .DELTA.n is greater than the degree of increase in the thickness "d", the birefringence (.DELTA.n d) is reduced.
On the other hand, a direction where displayed images are tinged with red is identical with a direction of a minor axis of liquid crystal molecules in bright state, in which case, the index anisotropy .DELTA.n remains unchanged, and the thickness "d" of a liquid crystal layer is increased. Eventually, the birefringence (.DELTA.n d) is increased.
FIG. 6 illustrates calculation results about how an apparent birefringence (.DELTA.n d) is varied as a viewing angle varies. An axis of ordinate in FIG. 6 indicates .DELTA.n d.times.2, which corresponds to a principal wavelength of spectrum of transmissive lights. The index anisotropy .DELTA.n and the thickness "d" of a liquid crystal layer both used for calculation were arranged so that a value of .DELTA.n d.times.2 is equal to 550 nm, when viewed from a front. The calculation was made on the assumption that liquid crystal sandwiched between upper and lower substrates uniformly rotate.
It is understood in view of FIG. 6 that an apparent birefringence becomes smaller if a viewing angle is inclined towards a major axis of liquid crystal molecules, and as a result, a principal wavelength of transmissive light spectrum deviates towards a shorter wavelength, and hence, displayed images are tinged with blue, whereas an apparent birefringence becomes greater if a viewing angle is inclined towards a minor axis of liquid crystal molecules, and as a result, a principal wavelength of transmissive light spectrum deviates towards a longer wavelength, and hence, displayed images are tinged with red.
Japanese Unexamined Patent Publication No. 9-80424 has suggested the use of birefringence medium for compensating for variation of birefringence in a liquid crystal layer, as a solution to the above-mentioned problem in an in-plane switching type liquid crystal display that displayed images are tinged with colors. FIG. 7 is an exploded perspective view illustrating the liquid crystal display suggested in the above-mentioned Publication. The illustrated liquid crystal display is comprised of a first compensation layer 25, a second compensation layer 26, and a liquid crystal layer 20, all of which are sandwiched between first and second polarizing plates 15 and 16. The first and second polarizing plates 15 and 16 are made of birefringence material. According to the Publication, optical axes of the birefringence mediums 25 and 26 are intersected at a front so that a phase difference between them is cancelled, and that variation in birefringence obtained when the birefringence mediums 25 and 26 are inclined is different from each other. As a result, the variation in birefringence obtained when the liquid crystal layer 20 is inclined can be compensated for.
However, it has been confirmed according to the optical simulation carried out by the inventors that the liquid crystal display suggested in the above-mentioned Publication did not have so superior viewing angle characteristic. FIG. 8 illustrates a volume-transmissivity characteristic of the suggested liquid crystal display, and FIGS. 9A and 9B illustrate loci of chromaticity change in the suggested liquid crystal display.
The first compensation layer 25 is composed of birefringence medium, and has the following indices of refraction. EQU ns=1.5850, nf=1.5800, nz=1.580
The second compensation layer 26 is composed of birefringence medium, and has the following indices of refraction. EQU ns=1.5845, nf=1.5820, nz=1.5810
Herein, ns and nf indicate indices of refraction associated with two optical axes parallel to a substrate, and nz indicates an index of refraction associated with an optical axis extending in a direction perpendicular to a substrate, namely, a thickness-wise direction of the first and second compensation layers 25 and 26. The first compensation layer 25 has a thickness of 100 .mu.m, and the second compensation layer 26 has a thickness of 200 .mu.m. The first compensation layer 25 is arranged in such a manner that a principal optical axis associated with the index of refraction ns is coincident with an optical axis of liquid crystal in bright state, and the second compensation layer 26 is arranged in such a manner that a principal optical axis associated with the index of refraction ns is perpendicular to an optical axis of liquid crystal in bright.
As is obvious in light of FIGS. 8, 9A and 9B, in the liquid crystal display suggested in the above-mentioned Japanese Unexamined Patent Publication No. 9-80424, displayed images are not released from being tinged with colors when viewing angles vary, but are tinged with colors to a greater degree.
In addition, as to the voltage-transmissivity characteristic, it was found that there occurred an inversion in gradation. That is, fluctuation in chromaticity caused by variation in a viewing angle leaves a big locus. In the voltage-transmissivity characteristic, the transmissivity increases at a front as a voltage increases, whereas the transmissivity decreases as a voltage increases in azimuth of an optical axis of the first or second compensation layer 25 or 26.
Japanese Unexamined Patent Publication No. 6-11714 has suggested a liquid crystal display capable of solving problems that displayed images are inverted in certain viewing angles, images cannot be seen at all, or displayed images are tinged with colors in a simple or active matrix liquid crystal display.
The suggested liquid crystal display includes a driving liquid crystal cell situated between a pair of polarizing plates, and having liquid crystal having orientation which is twisted when a voltage is not applied, and further includes an optical anisotropic device having an optical axis successively twisted. The optical anisotropic device has an optical axis of a surface through which a light enters, inclined by a certain angle relative to an absorptive or transmissive axis of a polarizing plate through which a light leaves.
However, according to the experiments the inventors had conducted, the suggested liquid crystal display cannot solve a problem that displayed images are tinged with colors in certain viewing angles.
Japanese Unexamined Patent Publications Nos. 2-285303, 4-16916, 432818, 5-27235, and 5-297223 have suggested STN liquid crystal display including an optical compensation layer wherein an index of refraction in a thickness-wise direction is greater than an index of refraction of at least one direction in a plane, to thereby improve a viewing angle characteristic. However, the viewing angle characteristic is not improved sufficiently to solve the problems of inversion of images and colored images.
Japanese Unexamined Patent Publication No. 6-11714 has also suggested a liquid crystal display including a driving liquid crystal cell situated between a pair of polarizing plates, and having liquid crystal having orientation which is twisted when a voltage is not applied, and further including an optical anisotropic device having an optical axis successively twisted. The optical anisotropic device has an optical axis of a surface through which a light enters, inclined by an angle .phi. (relative to an absorptive axis of a polarizing plate through which a light leaves. The angle .phi. is defined as follows. EQU .phi.=.DELTA.n.sup.2 .times.p.times.d.times.180.degree./4.lambda..sup.2
In the equation, .DELTA.n indicates optical anisotropy of the optical anisotropic device, "p" indicates a twisted pitch length of an optical axis of the optical anisotropic device, "d" indicates a thickness of the optical anisotropic device, and .lambda. indicates a wavelength of visible lights.
However, the suggested liquid crystal display cannot improve a viewing angle characteristic sufficiently to solve the problems of inversion of images and colored images.